1. Field of the Invention
The present invention relates to a method of protecting devices for the galvanization of metal products. It may be applied in a particularly advantageous manner to the continuous galvanization of steel sheet, in particular when such galvanization is only to be carried out on a single surface.
2. Description of the Prior Art
The following description is based on this latter case, but is given purely by way of non-limiting example.
It is known that the protection of steel sheet used in the automobile industry is often carried out by galvanization on a single surface, as the other surface is designed to be covered with a layer of paint.
The galvanization of a single surface may be carried out in several ways, for example by electrolytic deposition or by immersion of both surfaces and then removal of most of the deposit on one of the surfaces by electrolytic or mechanical means.
In accordance with other methods, a product, which prevents contact with zinc, is deposited on one of the surfaces before galvanization, and this product is removed after the galvanization operation.
In a prior patent, the applicants have proposed a device for galvanization of a single surface which may be used on existing galvanization lines with slight modification of these lines.
This method is characterised in that the sheet, before being contacted with a zinc bath, is wound about a rotary cylinder, partially immersed in the zinc bath. Contact of the sheet with the cylinder is carried out before contact of the sheet with the zinc on input to the bath, and is continued on discharge from the bath after the sheet - zinc contact has been interrupted, the surface adjacent to the cylinder not therefore being subjected to contact with the zinc.
In order to carry this operation out successfully, it is obviously necessary for the surface of the cylinder about which the sheet is partially wound to be treated in such a way that it is not reactive with zinc. In particular, the ends of the cylinder, which are not in contact with the sheet, should not be capable of being galvanised or removing zinc, so as to avoid zinc contacting the surface of the sheet to be masked during lateral movements of the sheet, which may not be avoided or during variations of the sheet width.
As far as the applicants know, no completely effective method has up to now been proposed in order to solve the problem to which the present invention aims to provide a particularly advantageous solution.